A fuel tank for a motor vehicle is usually provided with valves for operational venting and for refuelling venting. It will be noted at this point that the term operational venting is used herein to denote venting the fuel tank during operation of the vehicle, while refuelling venting is used herein to denote venting the tank during a tank filling or refuelling procedure.
Thus, during operation of a motor vehicle in which a tank is installed liquid fuel may at least partially change from the liquid state into the gaseous state, due to the effects of temperature or caused by swirl or surge movements of the fuel in the tank. In addition fuel can also expand due to the effect of an increase in temperature. Finally, fuel is also taken from the fuel tank during operation of the motor vehicle so that the fuel tank is usually not hermetically closed off in relation to the ambient atmosphere at least at certain pressures which can obtain in the tank. It will also be appreciated that the fuel tank is opened when refuelling same. It is then necessary for the gas which is displaced out of the interior of the tank by the incoming flow of fuel in the refuelling procedure to be discharged in an appropriate manner. Depending on the respective volume flow of fuel involved in the refuelling procedure, the amounts of gas involved may be up to 60 liters per minute. It will be appreciated that the hydrocarbon-charged gases which are displaced out of the interior of the tank in that way are not discharged to the atmosphere without being filtered. For that purpose either the gas produced in the refuelling procedure can be discharged to the atmosphere by way of a fuel vapor filter, as is usual for example with systems in the United States of America, or the gas involved may be sucked away by means of a specific item of equipment at the refuelling gun, as is employed in a European system.
Apart from the above-discussed operating conditions in which the fuel tank is required to communicate directly or indirectly with the atmosphere, it is necessary at least on a short-term basis to hermetically seal off the fuel tank for a procedure referred to as on-board diagnostics checking or OBD checking. In OBD checking the fuel tank is hermetically closed off and the interior thereof is subjected to an increased pressure or a reduced pressure, with respect to atmospheric pressure, for the purposes of checking sealing integrity. If an unacceptable pressure drop is found to apply the on-board electronic system of the vehicle records a leak in the tank.
Finally, an operation of cleaning the fuel vapor filter should also be mentioned as an operating condition which has to be suitably taken into account in terms of circuitry and control procedures. In the operation of cleaning the fuel vapor filter combustion air is sucked in from the atmosphere by the internal combustion engine of the motor vehicle in which the tank is fitted, by way of the fuel vapor filter.
All the above-mentioned operating conditions of a motor vehicle require the provision of suitable valve and control procedures and equipment on the fuel tank. For the purposes of controlling the operating conditions outlined above, ventilation valves for allowing air into the tank, vent valves for the discharge of medium from the interior of the tank, diagnostic valves and purge valves as well as a suitable system for appropriately controlling those valves need to be provided to implement the various operating conditions involved. In that respect, vent systems which have been employed hitherto are designed either for checking sealing integrity by means of a reduced pressure or for checking sealing integrity by means of an increased pressure. Various valves are provided for respective ones of the various operating conditions, the valves being arranged at spacings from each other and connected with conduits. Such systems are inflexible as diagnosis of sealing integrity is desired either by means of an increased pressure or a reduced pressure, depending on the respective type of vehicle involved and its on-board electronic system. In addition, conduits between valves and filters constitute potential sources of emission of gaseous hydrocarbons which are also unwanted.